Eight years after leaving Earth, the BepiColombo spacecraft has begun the final phase of its journey to Mercury, the solar system's innermost planet.
BepiColombo is a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency, launched in October 2018. Reaching Mercury is an unusually difficult engineering challenge, not because the planet is far away, but because it sits so deep in the sun's gravitational pull that a spacecraft arriving with too much speed will simply be pulled past it or into the sun rather than settling into orbit. To manage that, BepiColombo has spent years looping through the inner solar system, using gravity assists from flybys of Earth, Venus, and Mercury itself to gradually shed speed and line up for orbit insertion.
Once it completes this final approach, the mission is designed to split into two separate orbiters, one built primarily to study Mercury's surface and composition and another focused on its magnetic field and the thin envelope of particles surrounding the planet, known as its exosphere. Together, they are expected to build on the work of NASA's MESSENGER spacecraft, which orbited Mercury from 2011 to 2015, and the even earlier flybys by Mariner 10 in the 1970s, the first spacecraft to ever visit the planet.
Scientists are particularly interested in Mercury because its extreme proximity to the sun, its surprisingly large iron core relative to its size, and evidence of past volcanic activity make it an outlier among the rocky planets, offering clues about how planets form and evolve close to their host star.
Mission teams will be watching for confirmation of successful orbit insertion and the eventual separation of BepiColombo's two orbiters once it settles into position around Mercury.
Why does it take so long to reach Mercury?
Mercury sits deep in the sun's gravity well, so spacecraft must use a long series of gravity-assist flybys to slow down enough to enter orbit safely.
What will BepiColombo study?
Once in orbit, its two separate spacecraft will study Mercury's surface composition and its magnetic field and thin atmosphere, known as its exosphere.